Study the effect of arsenic on the immune response to Dukoral This study will be conducted on children (2-5 years) in arsenic contaminated area (Shahrasti) and arsenic free area (Mirpur). For this purpose, a total of 308 children (154 from each site) will be enrolled and vaccinated with Dukoral. This study will for the first time compare immune responses to a mucosal vaccine in an arsenic prone and arsenic low area. Urine will be collected from all participants, prior to immunization for determining arsenic levels.